The present invention relates generally to dosimetry and more particularly, to a dosimeter and method for using the same to detect and quantify a dosage of ionizing radiation.
It is important for those working with or near hazardous radiation sources to determine their level of exposure to the radiation, i.e. the radiation dosage. Situations can arise where workers must stop work and leave their work site when they have been exposed to a radiation dosage that exceeds a predetermined value. Dosimetry relates to the measurement of radiation dosages, and a dosimeter is a portable radiation sensor that provides the wearer with a radiation dosage and may identify the type or types of radiation that contribute to that dosage. Exposure of a dosimeter to radiation produces physical and/or chemical changes in the dosimeter. An analysis of these changes can provide information relating to the type of radiation to which the dosimeter has been exposed, the dosage of that type of radiation, the total dosage of several types of radiation, and other valuable information depending on the type of dosimeter. Some types of dosimeters provide the user with this information in xe2x80x98real-timexe2x80x99, i.e. immediately, so that the user can quickly determine whether to continue working or stop work and leave the area to avoid continued exposure.
Dosimeters usually include a radiation-sensing element enclosed within a protective housing. The housing may also act as a radiation filter, selectively absorbing some types of radiation but not others. A thin plastic housing, for example, can absorb alpha radiation but pass beta radiation. A cardboard, aluminum, or thick plastic housing can absorb both alpha and beta radiation, while a lead housing can absorb even more penetrating radiation such as gamma rays and x-rays.
Some types of dosimeters include or can be used with a mechanical meter to indicate the numerical value of a particular radiation dosage. Other types of dosimeters use the appearance or disappearance of a particular color to indicate that the dosimeter has been exposed to at least a predetermined threshold of radiation. These xe2x80x9ccolorimetricxe2x80x9d dosimeters may provide a range of colors and/or intensities of colors that are easily compared to a color chart to determine the radiation dosage. U.S. Pat. No. 3,899,677, for example, entitled xe2x80x9cPlastic for Indicating a Radiation Dosexe2x80x9d by Y. Hori et al., which issued Aug. 12, 1975, describes a colorimetric dosimeter. The dosimeter is produced by preparing a solution of a chlorinated polymer, a plasticizer, and at least one acid sensitive coloring agent, and then coating a support with the solution to form a film. Irradiation of the supported film produces hydrogen chloride, which reacts with the coloring agent to produce a visible color change. U.S. Pat. No. 3,691,380 entitled xe2x80x9cThreshold Value Dosage Meterxe2x80x9d by K. Hubner et al., which issued Sept. 12, 1972, describes another colorimetric dosimeter that detects 0.5-10 megarad dosages of ionizing radiation. The dosimeter is produced by preparing an aqueous solution of polyvinyl alcohol, methyl orange indicator, chloral hydrate, and sodium tetraborate (borax) buffer, and then coating a support with the solution to form a film. Irradiation of the supported film with ionizing radiation results in the production of hydrogen chloride, which reacts with the indicator to produce a color change. The borax buffer attenuates the radiation threshold of the dosimeter by reacting with initially generated hydrogen chloride. After the buffer is exhausted, any additional hydrogen chloride present reacts with the indicator.
A wide variety of radiation-sensing materials are available. Some are more sensitive to ionizing radiation than others. Silver-containing dosimeters are among the most sensitive since silver has a high cross section for many different types of radiation. The xe2x80x9cDT-60xe2x80x9d personal dosimeter, for example, includes a block of silver-activated phosphate glass housed in a plastic locket; it is described in more detail in xe2x80x9cRadiation Dosimetryxe2x80x9d, edited by F. H. Attix et al., Academic Press, (1966), vol. II, chapter 13, page 258. Silver can also be employed as an emulsion of microscopic silver halide crystals dispersed in gelatin. These emulsions are typically coated onto a support to form films. When such a film is exposed to radiation, an image is produced. The image can be in the form of particle tracks, and an analysis of the tracks can provide the identity and the energy of the particles that produced the tracks. Descriptions of dosimeters that employ silver emulsions can be found in xe2x80x9cRadiation Dosimetryxe2x80x9d, vol. II, chapter 15, and vol. III, chapter 28.
The chemical reactions resulting from the interaction of radiation and silver salt particles are well known for silver emulsions used in photography, and are summarized by equations 1, 2, and 3 below:
(AgX)n+radiationxe2x86x92(AgX)nxe2x88x92mAg0m+(X2)m/2xe2x80x83xe2x80x83(equation 1)
(AgX)nxe2x88x92mAg0m+(RH)nxe2x88x92mxe2x86x92Agn0+(HX)nxe2x88x92m+Rnxe2x88x92mxe2x80x83xe2x80x83(equation 2)
HX+Bxe2x86x92HBXxe2x80x83xe2x80x83(equation 3)
As equation 1 shows, radiation interacts with a silver halide salt grain (AgX)n to produce a halogen (X2) and an aggregate grain ((AgX)nxe2x88x92mAg0m) having both silver salt and silver metal (Ag0). Halide X is typically bromide, chloride, iodide, or a mixture thereof, and the radiation includes non-ionizing radiation such as visible and ultraviolet (UV) radiation, and/or ionizing radiation such as alpha-, beta-, gamma-, x-ray-, neutron-, electron beam-, and proton beam radiation. A reducing agent (RH), i.e. the photographic developer, rapidly reduces the aggregate grain to produce silver metal (Ag0), an oxidation product (R), and acid (HX). A buffer system is added to prevent the acid from lowering the pH of the emulsion and altering the activity of the developer, which would result in an underdeveloped or overdeveloped photographic image. The reaction of acid HX with base B of the added buffer system to form an acid-base complex (HBX) is shown in equation 3.
It is clear that very sensitive real-time dosimeters are highly desirable. Although dosimeters that employ silver as a radiation-sensing element are among the most sensitive, these types of dosimeters cannot be used to measure a radiation dosage in real-time.
Therefore, an object of the present invention is to provide a highly sensitive method of detecting ionizing radiation.
Another object of the present invention is to provide a highly sensitive dosimeter.
Yet another object of the present invention is to provide a dosimeter having a silver-containing radiation sensing element.
Still another object of the present invention is to provide a dosimeter that detects ionizing radiation while preventing interference from non-ionizing radiation.
Another object of the present invention is to provide a dosimeter that achieves the foregoing objects and also enables the user to measure radiation dosages in real-time.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
In accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention includes a dosimeter that detects ionizing radiation. The dosimeter includes a breakable sealed container. A solution of a reducing agent is inside the container. The dosimeter has an air-tight dosimeter body with a transparent portion and an opaque portion. The transparent portion includes a transparent chamber that holds the breakable container with the reducing agent. The opaque portion includes an opaque chamber that holds an emulsion of silver salt (AgX) selected from silver chloride, silver bromide, silver iodide, and combinations of them. A winding passageway in the dosimeter provides fluid communication between the transparent chamber and the opaque chamber.
The dosimeter may also include a chemical pH indicator in the breakable container that provides a detectable color change to the solution at a pH of about 3-10.
The invention also includes a method of detecting ionizing radiation. The method includes providing a dosimeter that detects ionizing radiation. The dosimeter includes a breakable sealed container with a solution of a reducing agent stored inside. The dosimeter also includes a silver salt (AgX) selected from silver chloride, silver bromide, silver iodide, and mixtures thereof. The dosimeter has an airtight body with a transparent portion for receiving the sealed breakable container and an opaque portion that includes an opaque chamber for receiving the silver salt containing emulsion and an opaque passageway that provides fluid communication between the transparent chamber and the opaque chamber. To practice the method, the dosimeter is exposed to ionizing radiation. After breaking the container and allowing the solution to flow through the opaque passageway to the opaque chamber and physically contact the emulsion, the solution is returned to the transparent portion of the dosimeter. A detected color change in the solution can be used to determine a radiation dosage.